A conventional inertial filter 100 is described below referring to FIG. 16. The conventional inertial filter 100 is placed in a fluid flow path to classify particles. The conventional filter 100 has a cylindrical filter body 101, and the filter body 101 has a through cavity 102 having a circular shape in cross section and penetrating through from an upstream side to a downstream side in a fluid passage direction. The through cavity 102 includes a diametrically-reduced through cavity 102a provided on the upstream side and having an inner diameter gradually smaller, and a diametrically-constant through cavity 102b continuous to the diametrically-reduced through cavity 102a on the downstream side and having an inner diameter dimensionally fixed. The diametrically-constant through cavity 102b is filled with a metal fiber 103 which is an example of incompressible fibers. The metal fiber 103 is secured by a mechanism not illustrated in the drawing so that the metal fiber 103 does not fall off from the diametrically-constant through cavity 102b and drop downward in the fluid passage direction.
According to the inertial filter 100, an internal pressure of the inertial filter 100 is lowered to or under an external pressure by a suctioning force of a pump not illustrated in the drawing so that a fluid flows in the through cavity 102 from the direction of arrow A to the direction of arrow B by a pressure difference resulting from the internal and external pressures to allow for classification of particles. The fluid increases its velocity of fluid flow in the diametrically-reduced through cavity 102a, and the velocity of fluid flow becomes constant once the fluid enters the diametrically-constant through cavity 102b. Then, fine particles contained in the fluid collide with and captured (collected) by the metal fiber 103 in the diametrically-constant through cavity 102b. 